Catalyst-mediated reactions of carbon dioxide represent one potential positive contributor to climate-relevant carbon capture and storage/sequestration (CCS). Well-designed reactions that utilize waste CO2 in the production of commercially relevant chemicals are much sought after. Some of these reactions include the formation of carbonates, where the carbonyl carbon obtained from CO2 is isohypsic with its starting material and does not require reagent driven oxidation state changes. The acid catalyzed cycloaddition of CO2 with an epoxide to form a cyclic carbonate, a functionality having various important applications, is a highly atom-economical reaction. Mechanistically, this reaction is based on an acid catalyst that activates the epoxide, which can then be attacked by a nucleophile co-catalyst to form an alkoxide. This intermediate can then react with carbon dioxide to give ultimately the cyclic carbonate. However, on account of the relatively inert nature and low reactivity of CO2, its activation and incorporation into organic substrates still remains a formidable challenge.
Although some homogeneous and several types of heterogeneous catalysts, such as zeolites, silica-supported salts, metal oxides, titanosilicate, a microporous polymer and an organic network have been utilized for the synthesis of cyclic carbonates, most of the processes demand high pressures and temperatures, thus requiring high energy and capital costs.
Ring-opening of epoxides with hydrides is one of the most fundamental reactions in organic chemistry and generally proceeds via an SN2 type mechanism. In the case of asymmetric molecules, hydride attack typically occurs at the most sterically accessible site to form the Markovnikov product. Anti-Markovnikov products of epoxide ring-opening, such as primary alcohols, are critical to the chemical and pharmaceutical industry and significant effort has gone into developing reagents and methodologies to obtain regioselective products. With metal hydrides, Lewis acids such as transition-metals, AlCl3, or BH3 are added to the reaction to shift regioselectivity to the anti-Markovnikov product. While extremely effective, these reagents are added in stoichiometric amounts and, in most cases, lack functional-group tolerance.